1. Field of the Invention
The present invention relates to an optical element used as an optical lens (e.g. opthalmic lens, camera lens), a filter, a polarizer, a semitransparent mirror, etc. and particularly to an anti-reflection optical element with reduced surface reflection.
2. Description of Prior Art
As the material for an optical element, there have conventionally been used mainly inorganic glasses. In recent years, however, plastics have gained wide acceptance as said material, because they are lightweight and superior in impact resistance, tintability, etc. Plastics are being increasingly used particularly as a material for opthalmic lenses because they have excellent tintability and can meet a requirement in fashion aspect, of using a large-sized frame and a color lens in combination.
As the plastics used as a material for optical element, particularly opthalmic lens, the resins obtained by cast-polymerizing diethylene glycol bis(allyl carbonate) (hereinafter referred to as DAC) are in general use. Polyurethanes which have a higher refractive index than the refractive index (1.500) (these refractive indexes are relative to the d-line of Fraunhofer lines and hereinafter referred to as nd) of the DAC resins and are lightweight, are also in use as a lens material, as described in, for example, Japanese Patent Publication Kokai (Laid-Open) No. 199016/1985. The polyurethanes have excellent impact resistance, good tintability, a high nd (1.56-1.64) and a small specific gravity (1.22-1.44), and accordingly are suitable as a material for a thin and light optical lens.
The surface reflection in optical elements causes reduction in transmittance of the optical system and increase in light not contributing to image information, thereby lowering image contrast. Therefore, optical elements made of an inorganic glass as well as those made of a plastic have, in many cases, an anti-reflection film thereon to reduce the surface reflection. In the optical elements made of a plastic, an anti-reflection film is formed usually on an abrasion resistance film provided on an optical component in order to improve the mar resistance. In this case, a foundation layer is provided as necessary between the abrasion resistance film and the anti-reflection film in order to improve the adhesion between the two films. Also there is a case wherein a foundation layer is provided on an optical component and an anti-reflection film is provided on the foundation layer.
Such an anti-reflection film is formed usually as a vapor-deposited film of metal(s) or metal oxide(s), and is largely divided into a single-layered anti-reflection film and a multi-layered anti-reflection film the latter being obtained by laminating a low refractive index film layer and a high refractive index film layer by turns. Both the single-layered anti-reflection film and the multi-layered anti-reflection film are required to have excellent optical properties (e.g. desired refractive index, optical uniformity, excellent transparency), excellent mechanical properties (e.g. excellent mar resistance, excellent adhesion) and excellent chemical properties (e.g. excellent acid resistance, excellent heat resistance).
As the anti-reflection film provided on an optical element made of an inorganic glass, there are known, for example, a multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained from a mixed material consisting of Zr and Ta (at least one of them takes an oxide form), disclosed in Japanese Patent Publication Kokai (Laid-Open) No. 4759/1971; a multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained from a mixed material consisting of Ta.sub.2 O.sub.5 and ZrO.sub.2, disclosed in Japanese Patent Publication Kokai (Laid-Open) No. 22704/1980; and a multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained by using ZrO.sub.2 as a material. Of these films, the multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained by using ZrO.sub.2 as a material is in primary use. Meanwhile, in optical elements made of a plastic, the formation of an anti-reflection film cannot be conducted at high optical component temperatures as in optical elements made of an inorganic glass, and there are used a multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained by using ZrO.sub.2 as a material, because the multi-layered anti-reflection film, even when formed at low temperatures, has excellent transparency and a high refractive index as disclosed in Japanese Patent Publication Kokai (Laid-Open) No. 11603/1981.
However, the multi-layered anti-reflection film comprising a high refractive index vapor-deposited film layer obtained by using ZrO.sub.2 as a material, when formed on an optical component (e.g. plastic) whose temperature cannot be raised relatively high, shows practically an unacceptable decrease in heat resistance with the passage of time. As a result, in conventional anti-reflection optical elements whose optical components is made of a plastic, there has been a problem that their optical, mechanical and chemical properties are deteriorated with the lapse of time because the heat resistance of the multi-layered anti-reflection film is deteriorated with the lapse of time.